U.S. patent number 3,840,407 [Application Number 05/263,835] was granted by the patent office on 1974-10-08 for composite porous electrode.
This patent grant is currently assigned to Textron, Inc.. Invention is credited to Robert L. Oliver, Harvey N. Seiger, Neng-Ping Yao.
United States Patent |
3,840,407 |
Yao , et al. |
October 8, 1974 |
**Please see images for:
( Certificate of Correction ) ** |
COMPOSITE POROUS ELECTRODE
Abstract
A composite porous electrode for use as a gas-diffusion
electrode in a battery assembly is formed by coating one surface of
a porous plaque of non-corrosive metal with a layer of hydrophobic
material, for repelling aqueous electrolyte solution, and coating
the opposite surface with a layer of electronically conductive
material dispersed in a hydrophobic binder therefor. The second
coating is formed of a plurality of successive, integral layers of
increasing concentration of electronically conductive material.
Principally, for larger sized electrodes, the second coating is
formed with a thickness differential from bottom to top to balance
hydrostatic pressure differentials in the electrolyte. A battery
assembly is provided in which a pair of porous electrodes are
secured on opposite sides of a consumable electrode with aqueous
electrolyte therebetween. The housing is formed with ducts and
cavities to direct electrochemically reactive fluid, such as
halogen gas, against the outer surfaces of the porous
electrodes.
Inventors: |
Yao; Neng-Ping (Sylmar, CA),
Oliver; Robert L. (Granada Hills, CA), Seiger; Harvey N.
(Granada Hills, CA) |
Assignee: |
Textron, Inc. (Providence,
RI)
|
Family
ID: |
23003429 |
Appl.
No.: |
05/263,835 |
Filed: |
June 19, 1972 |
Current U.S.
Class: |
429/400;
429/101 |
Current CPC
Class: |
H01M
12/065 (20130101) |
Current International
Class: |
H01M
12/06 (20060101); H01M 12/00 (20060101); H01m
013/00 () |
Field of
Search: |
;136/12FC,12R,86A,86D,20,22,30,31 ;29/182,182.2,182.5,187.5
;75/208,212 ;264/105,111-112,127 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Skapars; Anthony
Attorney, Agent or Firm: Nilsson, Robbins, Bissell, Dalgarn
& Berliner
Claims
We claim:
1. A composite porous electrode, comprising:
a porous metal body having a first surface for facing
electrochemically reactive fluid and a second surface for facing
aqueous electrolyte solution;
a layer of hydrophobic material on said first surface,
substantially inert to halogen, for repelling said electrolyte
solution therefrom;
a coating of electrically conductive particles dispersed in a
hydrophobic binder therefor on said second surface, the
concentration of said electrically conductive material in said
coating thereof being lower in regions spaced relatively close to
said second surface than in regions spaced further therefrom.
2. The electrode of claim 1 wherein said coating of electrically
conductive particles comprises at least a first region spaced
relatively close to said second surface having a relatively low
concentration of electrically conductive particles and a second
region formed integral with said first regions, spaced further from
said second surface, and having a relatively high concentration of
electrically conductive particles.
3. The electrode of claim 2 wherein said first region contains
about 35-65 weight percent of activated carbon as said electrically
conductive particles and said second region contains a higher
concentration of activated carbon than said first region, said
higher concentration being in the range of about 60-90 weight
percent activated carbon.
4. The electrode of claim 3 wherein said coating of said
electrically conductive particles includes a third region integral
with said second region, spaced further from said second surface
than said second region and containing a higher concentration of
activated carbon than said second region, said higher concentration
of said third region being in the range of about 80-99 weight
percent activated carbon.
5. The electrode of claim 4 wherein said third region is
substantially thinner than said second region.
6. The electrode of claim 1 including hydrophobic material coated
on said second surface and formed integral with said coating of
electrically conductive particles.
7. The electrode of claim 1 wherein said electrically conductive
material comprises activated carbon having a surface area greater
than about 900 m.sup.2 /gm and an ash content of less than about
1.5 weight percent.
8. The electrode of claim 1 wherein said porous metal body is
formed of a metal selected from titanium, zirconium, hafnium and
alloys thereof.
9. The electrode of claim 1 wherein said porous metal body is
formed of titanium.
10. The electrode of claim 1 wherein said metal body has a porosity
of about 25-55 percent.
11. The electrode of claim 1 in which said coating of conductive
particles has a porosity of about 40 percent to about 80
percent.
12. The electrode of claim 1 wherein said hydrophobic coating on
said first surface and said binder material comprise a
polyfluorolefin resin.
13. A porous composite electrode having a thickness differential
between the bottom and top edges thereof, comprising:
a porous metal body having a first surface for facing
electrochemically reactive fluid and a second surface for facing
aqueous electrolyte solution;
a layer of hydrophobic material on said first surface,
substantially inert to halogen, for repelling said electrolyte
solution therefrom;
a coating of electrically conductive particles dispersed in a
hydrophobic binder therefor on said second surface; and
means for making electrical contact to said metal body;
said electrode having a thickness of first magnitude along the
bottom edge and increasing therefrom to a thickness of greater
magnitude along its top edge.
14. The electrode of claim 13 wherein said increase in thickness is
substantially uniform along the length of said electrode.
15. The electrode of claim 13 wherein said coating of said
electrically conductive material is formed with said thickness
differential.
16. The electrode of claim 15 wherein the thickness differential of
said coating of electrically conductive material is about 3-14
percent per inch length thereof.
17. The electrode of claim 13 wherein said metal body is formed
with said thickness differential.
18. The electrode of claim 17 wherein the thickness differential of
said metal body is about 3-14 percent per inch length thereof.
Description
FIELD OF THE INVENTION
The fields of art to which the invention pertains include the
fields of electrochemical converters, fuel cells, gas-type primary
batteries and batteries incorporating halogen electrolyte
solutions.
BACKGROUND AND SUMMARY OF THE INVENTION
It is well established that pollution of the atmosphere occurs in a
large part as a result of automotive internal combustion engines.
It has been suggested to replace such engines with electric motors
powered by on-board batteries. However, the low energy densities of
present day batteries make such electric automobiles impractical as
a replacement for the internal combustion engine in automobiles. In
efforts to overcome these deficiencies, a number of exotic
electrochemical cells have been developed having energy densities
an order of magnitude greater than the common lead-acid battery.
Among these are the hydrogen-oxygen fuel cell, which is too
expensive for practical use, silver-zinc batteries which have
limited life and are very costly, and sodium-sulfur,
lithium-chlorine and lithium-sulfur batteries, which are costly,
operate hot and must be sealed from the atmosphere. Nickel-cadmium
and lead-acid batteries of improved design have also been
developed. Some of these batteries, such as the lead-acid battery
have a very low recharge efficiency at high rates of charge and are
only useful in hybrid systems in which the batteries are recharged
during operation. Other systems which have been discussed in the
prior art are aluminum-chlorine systems utilizing molten or fused
electrolyte, for example a molten eutectic salt of
aluminum-chloride-potassium chloride-sodium chloride. A number of
aqueous systems have been suggested utilizing a gas-diffusion
cathode in aqueous electrolyte solution and a zinc or alkaline
earth metal anode. Some older patents refer to accumulator
electrodes utilizing an electrolyte cation identical to the active
anode component. Illustrative patent disclosures of the foregoing
systems can be found in LeDuc U.S. Pat. Nos. 3,294,586 and
3,421,994, Zito, Jr. U.S. Pat. No. 3,285,781, Blue et al. U.S. Pat.
No. 3,408,232, Childs U.S. Pat. No. 3,445,292, Portail U.S. Pat.
No. 1,716,461, Oppenheim U.S. Pat. No. 1,588,608 and Stokes, Jr.
U.S. Pat. No. 2,796,456. Also of some interest are U.S. Pat. Nos.
3,040,115, 3,073,884, 3,455,744, 3,459,596, 3,507,700 and
3,514,334.
Even batteries which can be efficiently recharged at high rates
suffer harsh drawbacks since practical recharging would require
specially constructed recharging stations with very unusual
capabilities. For example, for recharging in less than 10 minutes,
the charging station would have to be vast and the cables large and
expensive. In order to recharge a 20 kilowatt hour system in 10
minutes, one would need a 120 kilowatt supply. If the output of the
battery is 80 volts, the required current would be 1,500
amperes.
A novel high energy density battery assembly is disclosed in patent
application Ser. No. 141,906, now abandoned, entitled AQUEOUS
ALUMINUM-HALOGEN ELECTROCHEMICAL CONVERTER, by H. N. Seiger and E.
L. Ralph, and in patent application Ser. No. 141,880, entitled
ACTIVATED ALUMINUM ANODE FOR ELECTROCHEMICAL CONVERTER, by H. N.
Seiger, both filed May 10, 1971 and assigned to the assignee of the
present application, the disclosures of which are incorporated
herein by reference. The battery assembly disclosed by these
applications may be mechanically "recharged," i.e., the reactive
components are simply replaced. Specifically, the battery assembly
utilizes a consumable aluminum anode and a cathode comprising
halogen diffused through a porous, electrically conductive
electrode, in an aqueous electrolyte solution. The halogen is
metered in accordance with load requirements, until the aluminum
anode is effectively exhausted. Thereupon, the anode and
electrolyte are simply replaced. Subsequently, aluminum and halide
can be recovered from the electrolyte solution to provide a closed
ecological cycle. The nature of the porous gas electrode is of
major importance to the efficient operation of such consumable
anode battery assemblies.
The use of a gas electrode is well known in general fuel cell
technology (see "Handbook of Fuel Cell Technology," by C. Berger,
Prentice-Hall, Inc., Englewood Cliffs, N.J., 1968). A gas electrode
(which can more generally be considered as a type of oxidizing or
fuel electrode) consists of a current collector having a porous
structure which divides the introduced reactive fluid from a liquid
electrolyte. The porous electrode also provides a large active
surface for the electrochemical reaction of the introduced fluid.
While the porous structure and the materials for the porous
electrode may vary depending upon the electrochemical system, the
electrode must be (1) impervious to the liquid electrolyte, (2)
inert to corrosion, (3) highly electrically conductive and (4)
catalytically active for the electrode reaction. Furthermore, the
weight and the volume of the electrode must be kept at a minimum in
order to achieve a system with high specific energy and high
specific power. Of interest, in this regard, is the report "Thin
Fuel Cell Electrodes," by K. V. Kordesch, Report No. 4, June 1,
1963-May 31, 1964, Contract No. DA-36-00, AMC-02314(E), Task No.
1C6-22001-A053-04.
The present invention provides a porous electrode which meets the
foregoing criteria. A thin, lightweight composite porous electrode
is provided which can be used in a highly corrosive environment,
which provides consistent uniform metering of the introduced
electrochemically active fluid and which performs with the high
degree of efficiency necessary to effective utilization of the
active material. Application of the present porous electrode is not
limited to fuel cell systems, but includes industrial and
commercial electrochemical cells which utilize a corrosive gaseous
or other fluid reactant.
The composite porous electrode is constructed by applying
successive layers of a combination of hydrophobic binder material
and electrically conductive particles, sufficient pressure being
applied after each deposition to provide good electrical contact
between the successive layers. Specifically, the present electrode
consists of a porous metal body having a first surface for facing
an electrochemically reactive fluid and a second surface for facing
an electrolyte solution. A coating of hydrophobic material is
applied to the first surface and a similar coating, but having
electrically conductive particles dispersed therethrough, is
applied to the second surface facing the electrolyte. The coating
of electrically conductive particles comprises a plurality of
successively applied layers having successively greater
concentrations of the conductive particles. In a particular
embodiment, an electrode of very high efficiency is provided by
grading the coating material, or the porous metal body, so that the
electrode has a thickness differential between bottom and top, the
electrode increasing in thickness from the bottom edge to the top
edge so as to compensate for hydrostatic pressure differentials in
the electrolyte.
A novel battery assembly is provided in which the porous and the
consumable electrodes are secured in a housing and spaced by open
frame members wherein the electrolyte is contained. The housing is
formed with walls which include space for the electrochemically
reactive fluid and which direct the fluid against the outer
surfaces of the porous electrodes.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a perspective view of a battery assembly incorporating a
porous electrode and consumable anode;
FIG. 2 is a perspective, exploded view of the battery assembly of
FIG. 1;
FIG. 3 is a schematic, cross-sectional view of a composite porous
electrode of the present invention;
Fig. 4 is a flow chart diagrammatically outlining the principal
method steps for preparing the electrode of FIG. 3;
FIG. 5 is a schematic illustration of the relationships between the
composite electrode, applied gas pressure and hydrostatic
electrolyte solution pressure for a uniform thickness
electrode;
FIG. 6 is a schematic illustration of the relationship between the
composite electrode, applied gas pressure and hydrostatic
electrolyte solution pressure for a graded thickness electrode;
FIG. 7 is a schematic, cross-sectional view of an alternative
graded electrode structure; and
FIG. 8 illustrates the voltage characteristics of a battery
assembly of the present invention as compared to the voltage
characteristics of prior art batteries.
DETAILED DESCRIPTION
Referring to FIGS. 1 and 2, a battery assembly 10 is illustrated
including a non-conductive, e.g., of plastic, housing having front
and rear walls 12 and 14 which are juxtaposed to sandwich a pair of
flat, thin porous electrodes 16 and 18 and a consumable aluminum
anode 20 between the porous electrodes 16 and 18. A pair of open
frame members 22 and 24 of plastic or other non-conductive
material, have their outer edges juxtaposed on opposite sides of
the aluminum anode 20 to space the porous electrodes 16 and 18
therefrom. The open frame members 22 and 24 thus provide
reservoirs, e.g., at 26, for enclosing electrolyte solutions
between the respective porous electrodes 16 and 18 on opposite
surfaces of the consumable aluminum anode 20. Each housing wall is
formed inwardly with a recess, e.g., 28 defining an enclosing ridge
30 and 32 to space the recessed portions of the walls from the
outer sides of the porous electrodes 16 and 18. Inlet conduits,
e.g., 34, are formed through the upper portion of each wall 12 and
14, terminating in inlet tubes 38 and 40. Referring only to the
rear wall 14, for simplicity, an electrochemically reactive gas,
such as chlorine, can be fed through the inlet tube 38, into the
recess 28. The frames are provided with cutout regions 46 through
which are formed openings 48 for the threaded shank of respective
screws 50 therefor, only one of which is shown. The screws 50 are
inserted through the openings 48, through similar openings 52
through the outer edges of the porous electrode 18 and then into
threaded openings 54 formed through the wall ridge 32. The porous
electrode is thus sandwiched between the open frame member 24 and
the housing wall 14.
The aluminum anode 20 is formed with a plurality of openings 56 in
quadrature array which coincide with similarly formed openings,
such as 58, 60 and 62, respectively, through the porous electrodes,
rear housing wall 14 and front housing wall 12. Screws 64 are
inserted through the quadrature openings 56, 58, 60 and 62, and
secured externally of the housing by nuts such as 66.
The result is a compact, securely held battery assembly 10. The
porous electrodes 16 and 18 are formed with annularly directed
upper portions 68 and 70, respectively, which extend upwardly out
of the housing 10 on one side thereof and which are formed in their
upper corners with openings, such as 72 coincident with openings 74
and 76 formed through the upper corners on one side of each of the
housing walls 12 and 14. The threaded shank of a bolt 78 is
inserted through the housing wall opening 74 through the upper
corner openings, e.g., 72, of the porous electrode (which is sized
to thread closely with the shank 78), through the corner opening 76
in the rear housing wall 14 and secured by a nut 80. The aluminum
anode 20 is also formed with an annularly extending upper portion
82, but terminating upwardly out of the housing 10 on a side
thereof opposite that of the porous electrodes 16 and 18. An
opening 84 is formed through the upper corner of the aluminum anode
20 coincident with similarly formed openings 86 and 88 through the
upper corners, on that side, of the housing walls 12 and 14. The
threaded shank of a bolt 90 is threaded through the front housing
wall opening 86, closely threaded through the aluminum anode
opening 84, through the rear housing wall upper opening 88 and then
secured by a nut 92.
The result is a compact, rigidly secured battery assembly.
Electrical leads 94 and 96 can be secured to the bolts 78 and 90
respectively to make electrical contact, via the lead 94, with the
two porous electrodes 16 and 18 and, via the lead 96, to the
aluminum anode 20. The assembly can be immersed in a solution of
electrolyte, the solution entering the spaces between the porous
electrode 16 and 18 and opposite surfaces of the aluminum anode 20
via the cut-away portions 46 of the spacing frames 22 and 24.
The foregoing structure represents one mode of construction for the
battery assembly. Other structures could be used, such as molded
assemblies.
In operation, chlorine, or other electrochemically reactive fluid,
is fed through the inlet tubes 38 and 40 into the recessed wall
portions, e.g., 28. The reactive fluid is thus applied to the outer
surfaces of the porous electrodes 16 and 18 which are constructed,
as will hereinafter be detailed, so as to meter the
electrochemically reactive fluid into contact with the electrolyte
solution. As the electrochemically reactive fluid contacts the
electrolyte, it dissolves in the electrolyte and finally diffuses
to the conductive particle surface for electro-reduction.
Electrochemical oxidation takes place at the consumable aluminum
anode 20 to form aluminum chloride which dissolves in the
electrolyte and generates electrons at the lead 96 to yield an
electrical current.
The specific nature of the consumable anode 20, whether of aluminum
or other metal, and the nature of the cathode reactant, whether
halogen, or otherwise, are not part of the present invention except
insofar as the selection of material with which to form the porous
electrodes 16 and 18 to accommodate the use of a corrosive
electrochemical reactant such as the halogens. While the present
construction finds its best use with gaseous electrochemical
reactant, such as chlorine, it can also be used with liquid bromine
at room temperature. Details of the nature of the electrolyte,
gaseous reactant and purity of the anode 20 can be found in the
above-noted pending patent applications, Ser. Nos. 141,906 and
141,880. In an exemplary system, a mercury alloyed aluminum anode
is utilized with a chlorine oxidant and an aqueous electrolyte
solution of ammonium chloride.
Referring to FIG. 3, the manner of construction of porous
electrodes 16 and 18 is illustrated. A porous metal plaque 98 is
provided having a porosity of about 25-55 percent. Such a plaque
can be formed by filling a die mold of known volume with a known
weight of powdered metal. In accordance with this invention the
metal is selected from the group consisting of titanium, zirconium,
hafnium and alloys thereof, and certain kinds of alloys known as
Hastalloy. The metal powder can have a particle shape ranging
between spherical and smooth angular, a particle size distribution
within the range of -100 +25 mesh, and bulk density within the
range of 1.5 to 5.6 grams per cc. The powder is then sintered in a
non-reactive atmosphere under such conditions as time, temperature
and pressure so as to yield a body having a desired porosity. A
method for forming such bodies is given in detail in Davies U.S.
Pat. No. 2,997,777, incorporated herein by reference.
The metal plaque is coated with electrically conductive material,
as will be described hereinafter, which provides an inert backing
for reactive gas distribution as well as for current collection.
The plaque is corrosion resistant to the gas and to the
electrolyte. In this regard, experiments with nickel, stainless
steel, tungsten, palladium, gold, tin and aluminum have all shown
various degrees of attack by an aqueous solution of ammonium
chloride when used with chlorine reactant and are therefore
unsuitable for long term use. The desired thickness of the porous
metal plaque will vary with the electrode size and should provide
for mechanical strength of the electrode. Typically a plaque of
0.030 inch thickness, with 40 percent porosity, is adequate for an
electrode size of 6 inches by 6 inches. The desired porosity and
pore size of the plaque are to a large extent a function of the
operating pressure of the gas electrode. For a gas pressure in the
range of 0 psig to about 5 psig, a suitable porosity and mean pore
size are 40 percent and 10 microns, respectively.
The porous metal plaque is sandwiched between two coatings 100 and
102, the coating 100 being solely a hydrophobic material and the
coating 102 consisting of a mixture of hydrophobic material and
conductive particles. The hydrophobic material serves to repel the
electrolyte solution and prevent flooding of the electrode by the
electrolyte. Generally the thickness of the hydrophobic coating 100
may vary between 0.0005 inch to about 0.002 inch. The coating 100
of hydrophobic material on one side of the plaque serves only to
repel electrolyte whereas the coating 102 on the other side, being
loaded with electrically conductive particles, provides the
interface, i.e., gas-electrolyte- conductive particles within the
layer 102 for the electrochemical reaction to take place.
The coating 102 is preferably formed from a plurality of layers,
successively applied and containing successively increasing
concentrations of electrically conductive particles. In the
specific example illustrated, the coating 102 includes a single
layer 104 of unloaded hydrophobic material, about 0.0005 inch
thick, with successive layers 106, 108 and 110 containing
electrically conductive particles dispersed therethrough. The
electrode can be constructed without the thin layer 104 of unloaded
hydrophobic material without significant detraction from its
performance. In any case, as will be described below, when forming
the layer 104, as well as the other layers 106, 108 and 110,
sufficient pressure must be applied thereto to assure good
electrical contact between the loaded layer 106 and the metal
plaque 98.
In a preferred form of the invention, layers 106, 108 and 110 have
a graded hydrophobic structure in that the furthest layer 110
contains a relatively high amount of electrically conductive
particles, wherein layer 108 contains a lower concentration thereof
and layer 106 contains the lowest concentration of electrically
conductive particles. In the electrode illustrated, layer 106
consists of equal weights of hydrophobic binder material and
electrically conductive particles (on a dry weight basis), layer
108 consists of 25 weight percent binder and 75 weight percent
electrically conductive particles, and layer 110 consists of 10
weight percent binder and 90 weight percent electrically conductive
particles. It is to be understood that as few as two layers may be
adequate. The number of layers is governed by a balance of cost,
performance and useful life of the system.
In the embodiment exemplified in FIG. 3, the plaque 98 is formed of
titanium and a lead 112 of non-corrosive, electrically conductive
metal, such as titanium, is attached to the rear side of the plaque
98 as hereinafter described.
As a hydrophobic binder, one can utilize any of the well known
inorganic or organic materials which can be dried and/or cured to
form a hydrophobic film and which is substantially inert to
halogen. In this regard, one should not have any substantial
portion of polyethylene or the like. One can utilize such inorganic
materials as clay or kaolin. As useful organic binders one can
utilize condensation-type or addition-type polymer forming
material, examples of which include: phenolformaldehyde resin;
polyamide resins, such as nylon and polymers obtained from
dimerized fatty acids; aromatic polycarbonates; polyether resins,
such as epoxy resins, polyethylene oxide, polypropylene oxide,
phenoxy resins, polyphenylene oxide resins, polyoxymethylene and
chlorinated polyethers; polysulfide resins; polysulfone resins;
polyurethane resins; silicone resins, such as polydimethylsiloxane;
amino resins, such as urea-formaldehyde resin,
melamine-formaldehyde resin; heterocyclic polymers, such as
polyvinylcarbazole; polybenzimidazoles and polybenzothiazoles; and
polyfluorolefin resins such as polytetrafluoroethylene,
polymonochlorotrifluoroethylene, polyvinylidene fluoride and
fluorinated elastomers.
The polyfluorolefin resins are preferred as being completely inert
to the electrochemically reactive fluid and to the electrolyte, and
particularly preferred is polytetrafluoroethylene, sold
commercially under the trademark Teflon.
Suitable electrically conductive particles include activated
carbon, graphite, charcoal, lignite char and carbon. Activated
carbon is particularly preferred as providing a desirable
electrochemically active surface, high electrical conductivity and
a mechanically strong bonded layer. Furthermore, the activated
carbon is readily homogenously dispersed in a methanol solution
containing aqueous dispersed binder and therefore can be applied by
simple spraying techniques. Exemplary activated carbon powder has a
surface area greater than 900 m.sup.2 /g and an ash content of less
than 1.5 weight percent. Such powders are superior to graphite
powder for example, which imparts substantially higher electrical
resistivity to the bonded layers. The size of the particles can be
graded and in the illustrated electrode, layer 106 utilizes -80
mesh particles, while layers 108 and 110 utilize -200 mesh
particles.
The concentration of electrically conductive particles within each
layer determines the porosity, thickness and pore size of the
composite layer structure, all of which are important parameters
for a gas-diffusion electrode. The composite layers must be thick
enough to provide a gas-electrolyte interface within the layers and
to prevent solution flooding, yet must be thin enough so that the
reaction sites are easily accessible to reactive gas and the
reaction products are easily removed. Optimum thicknesses, as well
as pore size and porosity can be determined for any of the
materials used. In the exemplified electrode, the preferred
particle loading for the composite layers are 140 mg/in.sup.2 for
layer 106, 110 mg/in.sup.2 for layer 108 and 60 mg/in.sup.2 for
layer 110. The corresponding layer thicknesses, before sintering of
the electrode, as described hereinafter, are approximately 0.011
inch, 0.010 inch and 0.005 inch for the layers 106, 108 and 110,
respectively. These thicknesses may be varied by .+-.20 percent
without introducing a significant difference in electrode
performance. The complete composite electrode illustrated has a
thickness in the range of about 0.055 inch to about 0.065 inch
after sintering, including about 0.030 inch for the porous metal
plaque. The porosity of the illustrated composite layers 106, 108
and 110, taken together, is in the range of about 55-65
percent.
FIG. 4 illustrates a fabrication procedure for the composite
electrode of FIG. 3. Initially, the titanium lead 112, about 0.007
inch thick, is attached by spot welding, or the like, to one side
of the titanium plaque 98. The plaque is then cleaned in an aqueous
solution containing 1 volume percent hydrofluoric acid and 15
volume percent nitric acid, ultrasonically rinsed in a
water-acetone solution and then dried at 15 .mu. Hg vacuum. A
solution is formed of methanol containing an aqueous dispersion of
polytetrafluoroethylene (60 percent solids, specific gravity 1.50
-- sold under the trade name Teflon 30 by DuPont). The methanol
solution is sprayed onto the front surface of the plaque 98, and
dried at 50.degree. C in 15 .mu. Hg vacuum to form layer 104.
Another methanol solution is prepared containing equal weights of
polytetrafluoroethylene and activated carbon powder and sprayed
onto layer 104. This solution is prepared by adding an aqueous
dispersion of polytetrafluoroethylene containing three parts by
weight thereof on a dry basis to a suspension of three parts by
weight of activated carbon powder in about 59 parts by weight of
methanol. The dispersion is kept under mild agitation to insure the
homogenous suspension of the carbon powder and to prevent the
agglomeration of the polytetrafluoroethylene. After application to
layer 104, the electrode is dried at 50.degree. C at 15 .mu. vacuum
and then pressed under 4,000 psi at 23.degree. C to assure good
electrical contact between the layers. As shown by the arrow 112 in
FIG. 4, the above step of applying a 50-50 weight percent
polytetrafluoroethylene-activated carbon suspension is repeated
followed by drying and pressing as above stated to form the desired
loading and thickness for the layer 106. Equivalently, the
sintering and pressing operations may be done simultaneously
providing the atmosphere is inert, such as argon, or if the
operations are carried out under vacuum.
Spray solutions for layers 108 and 110 are prepared in the same
manner as above, but the volume of aqueous dispersed
polytetrafluoroethylene is reduced to one part by weight of
polytetrafluoroethylene (on a dry basis) for a layer 108 and 0.33
part by weight of polytetrafluoroethylene for layer 110. A small
amount, e.g., 2 weight percent, of a wetting agent can be added to
the methanol solution for emulsion stabilization but is not
generally necessary. In each case, double applications of the
solutions are made as shown by the arrows 114 and 116 and the
sprayed layers are pressed under 4,000 psi at 23.degree. C after
each application.
The aqueous dispersion used to form layer 104 is thereafter applied
to the back side of the plaque to form the layer 100 of
polytetrafluoroethylene thereon having a thickness of about 0.001
inch.
Thereafter, the electrode is sintered at about 320.degree. C in 15
.mu. Hg vacuum for about 20 minutes. In place of vacuum, the
electrode can be sintered under a moderate pressure, e.g., 1,000
psi, in argon, or other inert atmosphere.
Electrodes fabricated as above were tested as a chlorine gas
electrode at a pressure of 1-4 psi against a calomel reference
electrode in an aqueous NH.sub.4 Cl solution. The open circuit
voltage of the chlorine electrode was +1.03 volt versus the
reference electrode as compared to a theoretical value of +1.05
volt. This clearly indicated that the composite electrode was inert
to the chlorine gas and that a reversible chlorine reaction had
been established at the composite electrode surface. The chlorine
electrode voltage dropped 0.54 volt when discharged at 1.0
amperes/in.sup.2 current density in an Al/Cl.sub.2 cell. This
corresponds to a resistance of 0.54 ohm/in.sup.2 electrode area
which is largely associated with the ohmic resistance within the
composite electrode structure. This structural resistance value is
comparable to that of other gas electrodes. Upon continuous
discharge of the chlorine electrode at a constant current density
of 0.5 amperes/in.sup.2, a constant chlorine electrode voltage of
+0.8 volt versus the reference electrode was maintained over a 3
hour period before polarization appeared at the chlorine electrode.
However, the performance of the composite electrode quickly
recovered when the electrolyte was replaced, demonstrating that the
polarization was associated with the electrolyte near the electrode
surface rather than with resistance of the composite structure. In
this manner, the composite electrode was repeatedly used for over
20 hours with no apparent performance deterioration. Furthermore,
the composite electrodes showed no apparent flooding by electrolyte
solution and bubble tests showed that gas bubble distribution was
uniform over the composite electrode surface.
In other tests, the electrodes were tested as the chlorine gas
electrode in an aqueous aluminum-chlorine cell, at a pressure of
1-4 psi, and gave excellent electrochemical performance. Referring
to FIG. 8, the voltage characteristics (solution IR included) of
such a battery assembly is compared with the characteristics of
typical nickel-cadmium and lead acid batteries as well as an
aluminum-chlorine battery using molten aluminum chloride-sodium
chloride-potassium chloride (66 percent: 20 percent: 14 percent)
electrolyte at 150.degree. C (J. Giner and G. L. Holleck:
Aluminum-Chlorine Battery, NASA Cr-154, March 1970). The foregoing
battery assembly had an open circuit voltage of about 2.5 volts
compared to reported values of 1.3 volts for a nickel-cadmium
battery, 2.1 volts for a lead acid battery and 2.0 volts for the
molten electrolyte battery. Voltage-current behavior of these
batteries are compared as indicated by the lines 130, 120, 122 and
124, respectively.
The foregoing fabrication techniques and electrode structures are
quite suitable for small electrode sizes, e.g., 1-4 square inches.
With larger electrodes, there is some decrease in efficiency as a
result of hydrostatic pressure effects in the electrolyte.
Referring to FIG. 5, the applied gas pressure and hydrostatic
electrolyte pressure are compared for a composite electrode 126 as
fabricated above, but having surface dimensions of about 6 inch
square. It is seen that the effect of the applied gas pressure is
uniform throughout the length of the composite electrode structure.
However, it is also seen that the hydrostatic pressure of the
electrolyte, exerted on the electrode surface, varies from a
relatively high pressure at the bottom of the electrode to a
relatively low pressure at the top thereof. This results in only a
partial utilization of the total electrode surface with excessive
gas permeation and bubbling at the electrode top and with decreased
gas permeation at the electrode bottom. This effect is of no
practical significance in small electrodes, say, 4 inches or lower,
but becomes more significant with larger sizes.
Referring to FIG. 6, a composite electrode 128 is shown which is
designed to overcome such hydrostatic pressure effects. The
electrode 128 is formed with a thickness differential between the
bottom and top edges so that the electrode increases in thickness
from bottom to top in conformity with the decrease in hydrostatic
pressure of the electrolyte solution. By such means, the effect of
the applied gas pressure is small where the hydrostatic pressure is
small and relatively large where the hydrostatic pressure is
relatively large.
In the electrode 128 in FIG. 6, the thickness differential is
obtained by applying the carbon-binder layers with a graded
thickness. This is accomplished by compacting the applied layers
under different pressures along the length of the electrode. For
example, with a 6 inch square electrode, the surface can be divided
into 3 regions each having a cross-sectional area of 2 inches
.times. 6 inches. The carbon-binder layer in the bottom section is
formed with a basic thickness which is the same as described with
respect to the electrode structure of FIG. 3. For the middle
section, the thickness of each of the applied layers is increased
by about 20-35 percent of the basic thickness and for the top
section the increase is about 35-50 percent of the basic thickness.
Therefore, with three carbon-binder layers of 0.011 inch, 0.010
inch and 0.005 inch, the graded composite electrode has an overall
carbon-binder layer thickness varying between 0.026 inch on the
bottom to about 0.039 inch on the top. Generally, the thickness
differential of the carbon-binder coating should be about 3-14
percent per inch length of electrode.
The grading of the layers can be made gradual rather than as a step
function in the course of spray coating the three regions. The
pressures for compacting the layers can also be varied to maintain
uniform electrical conduction therealong. For example, the top
section can be formed using a pressure of about 4,000 psi, the
middle section using a pressure of about 3,000 psi and the bottom
section using a pressure of about 2,000 psi. The result is a higher
gas pressure drop across the composite layer at the top and a lower
pressure drop at the bottom yielding a gas pressure profile along
the length of the electrode balancing that of the hydrostatic
pressure of the electrolyte.
A 6 inch .times. 6 inch electrode formed with a graded structure in
accordance with the foregoing showed an improvement in gas
distribution as compared to a non-graded electrode of the same
size, resulting in a high utilization of the electrode surface,
e.g., greater than 80 percent. An electrode so prepared was tested
in the aqueous aluminum-chlorine cell referred to above. The
results (solution IR included) are substantially the same as that
of the small electrode of FIG. 3, and can be shown by the same line
130 in FIG. 8. It is seen that the open cell voltage was 2.5 and
that the cell voltage drop mainly resulted from ohmic polarization
due to solution resistance and structural resistance of the
chlorine electrode. When tested against a calomel reference
electrode, the composite electrode voltage dropped by only 0.5 volt
when discharged at 0.77 amp/in.sup.2. The resistance was primarily
confined to the porous structure of the composite electrode and was
approximately 0.65 ohm/in.sup.2. The performance of the larger
electrode therefore substantially reproduced the results obtained
with the smaller electrode illustrated in FIG. 3.
Referring to FIG. 7, there is illustrated an alternative embodiment
of the invention wherein a thickness differential is accomplished
by varying the thickness of the metal body 132 of the electrode 134
while maintaining the carbon-binder layers at a uniform thickness.
The same thickness dimensions as previously referred to can be
utilized. One may also vary both the thickness of the porous metal
body and carbon-binder layer to accomplish the same gas pressure
profile.
While the invention has been described with respect to a particular
electrode structure, various modifications, other than those
illustrated, can be made. For example, the thicknessess of the
composite layers and the compaction pressure may be varied to
obtain an optimum composite electrode structure for a specific
requirement of the electrode.
* * * * *